It is becoming more common for contactless power to be transmitted to portable electronic devices, for example by utilizing inductive coupling. Many inductive power supply systems suitable for powering portable devices include two main components: (1) an inductive power supply or primary unit having at least one primary coil, through which it drives an alternating current, creating a time-varying electromagnetic field, and (2) a portable electronic device or secondary device, separable from the primary unit, including a secondary coil, which when placed in proximity to the time-varying field, the field induces an alternating current in the secondary coil, thereby transferring power from the primary unit to the secondary unit.
Contactless power supply systems are not 100% efficient. That is, some energy is lost in order to transfer power from the primary unit to the secondary unit. For example, some losses may be caused by the switching circuit components and other losses may be caused by the primary coil, sometimes referred to as ohmic losses, which are proportional to the ohmic resistance in the components and to the square of the current running through them. Foreign objects, and especially metal foreign objects, can also affect efficiency and in some cases cause a safety concern. Metal placed in the field is sometimes referred to as parasitic metal. Some parasitic metal in the field may be acceptable, for example, many portable devices, even ones powered by contactless power supply systems, sometimes include metal. The acceptable metal is sometimes referred to as known or friendly parasitic metal.
Some systems and techniques have been developed to attempt to detect whether there is an unacceptable amount of parasitic metal in the field. One basic system includes a power consumption detector in the electric circuit of a power sending terminal. When a piece of metal is placed on the power sending terminal instead of a portable device, the amount of consumed power at the power sending terminal increases abnormally. In order to prevent this abnormality, the power consumption detector measures the amount of power consumed by the power sending terminal. When the measured amount of the consumed power reaches a predetermined upper threshold, it is determined that there is an unusual situation and transmission of power is suppressed. Although a system such as this provides basic parasitic metal detection, it has flaws. For example, the system cannot account for (1) friendly parasitic metal, (2) portable devices that consume different amounts of power, or (3) power losses due to the misalignment of the power sending terminal and the portable device.
Other parasitic metal detection techniques have also been developed. For example, some systems can account for (1) the power being supplied to the actual load of the secondary device, (2) the friendly parasitics of the secondary device, (3) situations where there is not a simple 1:1 relationship between the primary unit and the secondary device, or (4) situations where presence of the secondary device does not necessarily physically exclude all foreign objects. Some of these techniques involve disconnecting the secondary load or communicating information from the secondary device to the primary unit. A number of these techniques are described in U.S. Patent Publication 2007/0228833 to Stevens, filed on May 11, 2005 entitled “Controlling Inductive Power Transfer Systems” which is herein incorporated by reference in its entirety.
Although some previous systems can provide parasitic metal detection, in some situations these systems can be inadequate. For example, known systems do not account for the known losses accurately enough and therefore incur too many false positives that result in a system restriction or shut down. To put it another way, one issue with some known parasitic metal detection systems is that their resolution is loose enough that a piece of metal could heat up to an undesired level. Utilizing a method that has an improved resolution or accuracy to detect losses can address this and other issues.